P0171 Fuel Trim Issue in Dodge Truck After Intake Manifold Gasket Replacement

The issue continues with my Dodge truck. After replacing the intake manifold gasket (a painful job!), I drove it around town this weekend without any problems. However, during a long trip last weekend—where I hauled a light trailer for approximately 750 miles—the check engine light activated and symptoms worsened significantly. About one-third of the way through the journey, I noticed severe performance issues: fuel efficiency dropped to just 7 mpg, there was a noticeable lack of power even on gentle inclines, and black exhaust smoke began emitting from the tailpipe. The mechanic is currently investigating. He reported that negative fuel trim values return when carb cleaner is sprayed into the intake—this suggests an underlying leak or vacuum issue. Given this behavior, I suspect a persistent vacuum leak or a faulty gasket, possibly related to the intake system. I'm at a loss as to what's causing it. The symptoms point toward either a loose or improperly torqued component, such as the plenum pan or intake manifold gaskets. Could it be that the previous repairs weren't completed correctly? I'm wondering if the parts used were OEM Chrysler components and whether the repair followed the official TSB guidelines. Any insights into similar cases with P0171 in Dodge trucks would be greatly appreciated.